Basins

Basins – A basin is “a depression, in the sea floor, more or less equidimensional in plan and of variable extent” (IHO, 2008). In this study basins are restricted to seafloor depressions that are defined by closed bathymetric contours. Basins were mapped based on the identification of the most shoal, closed, bathymetric contours, examined regionally for the major ocean basins and shelf seas. Basins of the major oceans are nominally bounded by the foot of slope and by the mid-ocean spreading ridges (Wright and Rothery, 1998; Gille et al., 2004). However, numerous smaller basins of the bathyal and hadal zones, located outside of the major ocean basin areas, were mapped separately, again by identification of the most shoal, closed, 100 m, bathymetric contours. At abyssal depths we distinguish between major ocean basins, which are large basins (>800 km2), and small basins (<800 km2). The depths of major ocean basins (defined by the shoalest closed contour that they contain) illustrates that the major ocean basins in the Northwest Pacific are the deepest, at 5,300 m.

Basins mapped by Harris et al., (2014). The numbers indicate depths of major ocean basins based on the most shallow, closed, bathymetric contour that defines the basin outline, illustrating that the deepest basins are located in the northwest Pacific.

We also identified basins perched on the slope, again mapped by identification of the most shoal, closed, 100 m, bathymetric contours that defined a discrete basin. Basins perched on the Antarctic shelf were mapped by identification of the most shoal, closed, 50 m, bathymetric contours that defined a discrete basin (see glacial troughs). Basins perched on the rest of the world’s shelf areas were mapped by identification of the most shoal, closed, 10 m, bathymetric contours that defined a discrete basin. These included the basins within shelf seas, glacial troughs and fjord basins found in the higher latitudes. A key point about basins is that they overlay not only the basal layers, but also other features (i.e. other individual features may occur partly or wholly within a basin or basins). Basins cover the greatest area across all feature layers, equal to 158,529,660 km2, or 43.8% of the oceans.

Map of basins (cross-hatched) overlain with bottom water dissolved oxygen (DO; taken from NOAA World Ocean Atlas), showing how basins connect zones of high bottom water DO adjacent to source areas in Antarctica and low DO in the northeast Pacific. Arrows suggest possible pathways of bottom water flow.

Basins and bottom water

The formation of cold, dense water that occupies the abyssal layer of the global ocean occurs in the polar regions of the earth and is driven by two major processes: 1) the cooling of Gulf Stream waters around Iceland to form North Atlantic Deep Water; and 2) the formation of bottom water by sea ice formation and brine rejection. North Atlantic Deep Water (NADW) is formed as cold surface waters during the winter in the Greenland and Norwegian Seas. This water sinks to fill the basin north of a ridge spanning the distance from Greenland to Scotland.

The coldest bottom water, Antarctic Bottom Water (AABW), is created in winter on the continental shelf of Antarctica by sea ice formation and it is about -1 oC; this highly oxygenated water fills basins in the Pacific Ocean. As the water moves northwards it gets warmer and is gradually depleted of oxygen until it reaches the northeast Pacific. At this point, at the end of the global ocean conveyor, it contains less than 2 ml/l dissolved oxygen.

Basin statistics (from Harris et al., 2014). The percentage areas refer to the percentage of ocean region that is basins, except for basins perched on the slope, where the second number (in brackets) refers to the percentage of slope area that is occupied by basins, or for basins perched on the shelf, where the second number (in brackets) refers to the percentage of shelf area that is occupied by basins.